The present invention relates to an actuator of the type known from DE 10 2005 038 891 A1 and also from DE 10 2011 052 528, which was not previously published.
It is known from this prior art (as well as from the fundamental general technology, which is assumed to be known) to use magnetically active shape memory alloy materials (MSM materials=Magnetic Shape Memory, equivalent here to SMA: Shape Memory Alloy) for the actuator system. For this purpose, for instance, a magnetic field generated by an energised coil is applied to an MSM (SMA) crystal body (as a representative for a multiplicity of possible MSM-based materials), produced for instance on the basis of an NiMnGa alloy. As a reaction to exposure to a magnetic field in this manner, the MSM crystal body executes an expansion movement in a direction perpendicular to the magnetomotive force direction with the electromagnetically generated field. This expansion movement can then, at a driven end, drive an actuation partner that interacts with said end.
Such magnetically active shape memory alloy materials and actuators (actuator devices) implemented in the basic manner described thus offer a possibility of replacing or supplementing customary magnet-based actuator principles (such as electromagnetic actuators). The advantage of the magnetic shape memory alloy material used, in addition to simplicity of mechanical design in implementation (no armature moves as a whole; only an expansion of the expansion unit takes place), is most of all a potentially fast reaction time of the expansion to the application of the magnetic field (at the necessary level).
However, MSM-based actuator devices, which are assumed to be known, also have disadvantages (owing to the basic principle and design) compared to other actuator principles, as a result of which actually universal usability has been limited in practice. For instance, a usable travel of the expansion movement (i.e. an extent of an extension movement executed by the MSM crystal) is typically limited to approx. 3% to 6% of a corresponding axial extent of the crystal, so straight, large-travel movements can be implemented only with difficulty by means of SMA/MSM actuators.
Furthermore, known MSM (SMA) actuators have the property of the intended expansion movement taking place as a reaction to an applied magnetic field (of a necessary minimum field strength). After a drop in the magnetic field (below the minimum threshold), there is however no automatic compression back to the originally compressed state (the starting position). Rather, the crystal consisting of the SMA material remains in the expanded position even after the drop below an expansion threshold or the complete deactivation of the magnetic field. With devices known from the prior art, such as DE 10 2005 038 891 A1, which is cited as the generic type, resetting (i.e. returning the expansion to the non-expanded starting position) is therefore implemented with restoring means, which are formed in said document as a mechanical spring. Such a spring exerts its spring force counter to the expansion direction. If the spring force of such a known restoring spring in relation to an (unloaded) expansion force of the SMA crystal is set up in such a manner that the SMA material overcomes the counter force (restoring force) of the spring on exposure to the magnetic field, the intended expansion movement takes place. When the magnetic field drops (below the necessary minimum field strength) after the end of energisation, the spring force (restoring force) acting on the MSM crystal then results in contraction back to the starting position.
A suitable combination of the SMA (MSM) crystal characterising the expansion unit as the SMA material in interaction with the suitably dimensioned restoring spring thus achieves a desired actuation and restoring behaviour, so numerous actuation tasks can be carried out with technology of this type.
However, the use of a spring (for instance a helical spring, as disclosed in DE 10 2005 038 891 A1) has the disadvantage of a typically linearly ascending characteristic in the force-path graph, that is, in the increasingly expanded state of the expansion unit (that is, with increasing expansion travel), the restoring spring force acting on the expansion unit increases. This property is disadvantageous precisely with regard to a desirable actuation force of a generic actuator device, said force being constant over the entire actuation travel (expansion travel), since it regularly results in the actuation force dropping considerably shortly before the end of the effective travel maximum, and therefore some actuation tasks cannot be implemented reliably. The work (as a difference between MSM actuation force and spring restoring force over the effective travel range) that can be realised by such a device (i.e. combination of SMA-based expansion unit with restoring spring) is also limited correspondingly thereby.
DE 10 2011 052 528 of the applicant already notes this fundamental disadvantage of the technology forming the generic type and achieves the resulting object of equalising the (resulting) actuation force over the entire expansion travel by using as the restoring means a spring combination or spring configuration, the restoring force behaviour of which does not ascend linearly over the expansion profile but drops or has a flat profile, at least in some sections. As a result, the actuation force acting on a downstream actuation partner also increases in expanded travel sections.
However, this procedure is also mechanically complex, demands careful setup of the spring units to be provided in this case as restoring means and in this respect has potential for further improvements.
It is therefore the object of the present invention to improve a generic actuator device with regard to the (actuation) force behaviour thereof along the entire movement travel and at the same time to ensure reliable restoration by the restoring means. In particular, an alternative to the restoring springs, which are assumed to be known, must be provided with regard to the restoring means according to the invention.